This invention relates to a method and device for providing a sheet winder of the type using a touch roller with automatic control which enables the contact pressure of sheet roll upon the touch roller, a parameter that determines the quality of sheet rolls produced, to be constantly maintained at the optimum magnitude.
In the production of rolled sheets of various materials such as paper, synthetic resins and cloths, there is acutely felt the need for a sheet winder capable of minimizing the unavoidable deformation and dimensional change of the sheet material which occurs in sheet winding and of permitting production of quality sheet rolls the sheet material of which retains the properties which it is intended to possess. One of the existing sheet winders claimed to be capable of producing sheet rolls of high quality has a means called a touch roller with which the roll is held in contact immediately behind the point of contact between the sheet being wound and the underlying roll. This touch roller serves to press the sheet against the rotating roll so as to force out any air entrapped between the sheet and the surface of the underlying roll for the purpose of obtaining a sheet roll having the sheet wound up to the density most suited to the particular material of the sheet. The touch roller which concurrently serves as a guide for the sheet is usually set in such a position that the sheet roll being formed rides on the upper part of the touch roller. Owing to this contact, the touch roller rotates at the same peripheral speed as the sheet roll while the sheet is being wound round the sheet roll. The core of the sheet roll moves away from the touch roller as the winding of the sheet progresses and the sheet roll grows gradually in diameter. If this process is left to proceed freely, the contact pressure of the sheet roll upon the touch roller will depend solely on the weights of the roll and the rocking arm on which it is supported and will increase in proportion to the amount of the sheet wound in the sheet roll. Consequently, the contact pressure near the end of the winding operation will be vastly different from the contact pressure at the start of the sheet winding. This method, therefore, is incapable of providing the optimum contact pressure throughout the winding operation and is therefore unsuitable for the winding of sheet material and is particularly unsuitable for winding thin sheet material which is apt to elongate.
An improved sheet winding method is known in which a force is applied to the sheet roll in the direction serving to raise it from the touch roller. This method effects automatic control of the contact pressure by detecting the radius of the sheet roll as an indication of the amount of sheet wound thereon, determining the desirable contact pressure for the amount of wound sheet detected on the basis of empirical data and, applying a force to the arm supporting the sheet roll to raise the sheet roll away from the touch roller to the degree required to obtain the optimum contact pressure between the sheet roll and the touch roller. Although this method has proved satisfactory in the winding of thick sheet material which does not easily elongate under tension, it fails to provide the precise control required for the winding of a sheet of thin material which is easily deformed. In this method, the amount of sheet already wound up is determined from the radius of the wound sheet roll which is in turn determined from the distance between the axis of the touch roller and the axis of the sheet roll. Since the contact pressure between the touch roller and the sheet roll causes a depression in the surface of the sheet roll, the distance between these axes does not faithfully reflect the radius of the wound sheet roll so that an error is introduced in the determination of the optimum contact pressure. Even if this disadvantage should be eliminated somehow, there still remains the fact that the characteristics of the optimum contact pressure have never been established with due consideration to the properties of the particular material of the sheet to be wound up. Thus, this method can hardly be expected to provide definitely reliable control of the sheet winding.
The need for a method capable of producing rolled sheets of high quality is felt in various industries. In searching for a way to meet this need, the inventor took particular interest in the immense effect produced on the quality of finished sheet rolls by the contact pressure between the sheet roll and the touch roll and, through systematic analysis of a huge volume of technical data which he collected, determined the optimum contact pressure for sheet materials having various properties and succeeded in developing a sheet winding method making effective use of the determined optimum contact pressures (Japanese Patent Public Disclosure No. 50549/1975, Patent Publication No. 44999/1977 dated Nov. 11, 1977).
The quality of a sheet roll hinges upon the winding density which in turn is affected by the tension applied to the sheet in the course of the winding. The sheet being wound is distorted or stretched when this winding density is higher than is normally required. When the winding density is too low, the sheet suffers from the "air table" phenomenon in which the sheet slips on the surface of the roll, the sheet edges fall out of alignment or the sheet roll sustains winding squeeze.
With a view to eliminating these defects, the inventor conducted an elaborate study on the properties of sheet material and, beginning from the basic principle that a comparatively thick sheet (such as of thick paper) which does not easily elongate and which has a non-slippery surface should be wound rather tightly (with high winding density) and a comparatively thin sheet (such as of synthetic resin film) which elongates easily and has a slippery surface should be wound rather loosely (with low winding density), he established optimum winding conditions for various sheet materials and developed the method for controlling the contact pressure on the touch roller as described above. In this method the radius of the sheet roll is continuously monitored by a measuring arm so ligth as to make substantially no depression in the sheet roll. Thus, with a means for accurately determining the amount of sheet wound on the roll and by controlling the contact pressure to the optimum contact pressure determined as described above, this previously developed method made it possible to produce sheet rolls of high quality.
Despite the advantages described, this method has one weak point in that it cannot be applied in unmodified form to conventional sheet winders and the construction of the device used for the operation of this method tends to become rather complicated.